1. Field of the Invention
The present invention relates to an optical fiber sensor and to a detection device provided with at least one sensor of this type.
It is known that optical fibers offer outstanding advantages in the transmission of data by light waves by means of a wide range of different sensors. The invention is concerned with the particular case in which the sensor is constituted at least partially by a sensitive zone of the fiber itself. A number of sensors of this type can be placed in series along the fiber and serve to collect information by means of their sensitive zones which usually consist of curved portions (bends) of the fiber.
2. Description of the Prior Art
Optical fibers are already in common use in measuring techniques. In particular, European patent No. EP-A-0 000 319 filed in the name of Battelle Memorial Institute presented a device based on optical fiber technology for use as a refractometer or level indicator. In this device, a light beam is injected in a predetermined direction into one end of the optical fiber in order to permit reflections of the light rays along the fiber, said optical fiber being immersed in a liquid having a refractive index which is different from that of the fiber. When the light rays which undergo these different reflections exceed a limit angle with respect to the surrounding medium, the light rays which have a smaller angle of incidence are refracted outside the fiber. When a curved surface is encountered along the fiber, the reduction in the angle of incidence of the light rays which impinge upon the surface produces an increase in the limit angle of the light beams with respect to the refractive index liquid, thereby causing the incident light rays which exceed the limit angle to pass by refraction within said liquid. By means of a combination of alternate curves or so-called bends arranged in succession in opposite directions, it is possible to obtain a light signal which has particularly high sensitivity and performs a function of amplification of the effects obtained at the time of passing through the first bend. This device permits applications in particular in the measurement of a level of liquid but requires an optical installation at each end of the fiber. It is also a common practice in techniques of measurement by electric fibers to subject the fiber to modifications of shape in order to permit measurement of certain parameters. Thus U.S. Pat. No. US-A-4,436,995 filed in the name of General Electric Company presents optical fiber sensors for measuring the amplitude of certain parameters such as mechanical displacements, pressure, temperature, current or voltage. To this end, a portion of the fiber corresponding to a sensitive zone is subjected to variations of bend curvature representing the change in the parameter to be measured. The losses resulting from microbends in the fiber represent an attenuation of the light energy between a light-emitting source and a light-arrival detector. In this device, the fiber is subjected to variations in bend curvatures by application of two teeth placed on each side of the fiber so that the throat of one tooth cooperates symmetrically with the crest of the opposite tooth, the bend curvature of the fiber being varied by clamping between said opposite teeth. As a result of this variation, detection of parameters can be obtained when light energy is injected into one end of the fiber and when the beam is scanned at the other end.
More specifically, the present invention relates to devices of this type in which the light signals are emitted at one end of the fiber and in which the device for receiving information is also placed at the same end.
The phenomena utilized in these devices no longer involve direct transmission of light from one end of the fiber to the other but the received items of information are contained in a so-called backscattered signal which propagates in the direction opposite to the emission light pulse. The light pulse emitted at one end of the fiber experiences losses during travel since part of the light is scattered and escapes from the fiber. The return signal received at the emission end therefore undergoes attenuation which increases with the distance traveled within the fiber. The phenomena detected by the sensitive zones of the fiber are represented by variations in attenuation of said back-scattered signal. It is apparent that these variations in attenuation considered alone are very small since they apply only to a minor proportion of the total light.
The aim of the present invention is to retain all the advantages of sensors employed heretofore by utilizing the variation in attenuation of the back-scattered signal received by a device placed at the same end of the fiber as an emission device while nevertheless permitting a signal of much higher strength.
The basic concept lies in the fact that the sensitive zone is placed in a coupling medium or so-called couplant, that the lost light which escapes from said sensitive zone through means such as a bend or in any other manner will propagate in said couplant and that a retroreflecting surface is employed for recoupling the losses within the fiber.